1. Field of the Invention
The present invention relates to a superconducting magnet and a method of producing the same.
2. Description of Related Art
Commonly used superconducting magnet is generally classified into three different types: a magnet made of an alloy superconducting wire material represented by a niobium-titanium alloy; a magnet made of a compound superconducting wire material represented by a Nb3Sn compound; and a magnet made of a combination of the above two types. For a given application, those three types are selected as appropriate in accordance with their characteristics.
The alloy superconducting wire material has a good mechanical property as compared with the compound superconducting wire material, and thus it maintains the superconductivity until the stress reaches a yielding level of 250 to 300 MPa. Moreover, it can satisfactorily withstand a bending strain of up to 2%. Therefore, when an alloy superconducting wire material is wound into a coil to produce a magnet, it is possible to perform tightly wind the wire material without any interstice between adjacent windings under application of a tension to the wire, thereby facilitating production of a magnet from the wire material. For such a ground, the majority of previous superconducting magnets have been produced from the alloy wire material.
However, an alloy superconducting wire material does not exhibit a sufficient superconductivity, particularly a sufficiently high critical current at the critical magnetic field, or at a high magnetic field, as compared with the compound superconducting wire material. Thus, a superconducting magnet made of an alloy superconducting wire material exhibits a lower magnetic field than does a superconducting magnet made of a compound superconducting wire material. For example, a superconducting magnet made of a niobium-titanium alloy wire material, which is most commonly used as an alloy superconducting wire materials, generates, at the maximum, a magnetic field of 8-9 T (one T equals to 10000 Gaus) in the presence of a liquid helium maintained at 4.2K, and a magnetic field of 11--12 T even in the presence of superfluid liquid helium maintained at 1.8K. Accordingly, a superconducting magnet made of an alloy superconducting wire material would not meet the desired specification particularly for applications which require an even higher magnetic field, such as a magnet for a high-resolution magnetic resonance imaging (MRI) device, or a high field magnet for the measurement of physical properties of an object.
On the other hand, a superconducting magnet made of a compound superconducting wire material exhibits a high critical magnetic field as compared with a superconducting magnet made of an alloy superconducting wire material. For example, a magnet made of an Nb3Sn wire material generates, at the maximum, a magnetic field of about 18 T in the presence of liquid helium maintained at 4.2K, and a field of about 21 T when exposed to superfluid liquid helium of 1.8K, which are far above the maximum magnetic field expected from a superconducting magnet made of an alloy superconducting wire material. Therefore, a compound superconducting wire material is generally considered to be most suitable as a raw material for a superconducting magnet which allows the generation of a magnetic field of not less than 12 T. However, because a compound superconducting wire material is composed of a intermetallic compound, it is susceptible to mechanical stresses: its yielding stress is 150 MPa, or should be taken as 100 MPa for design purposes, and its tolerable bending strain should be taken as about 0.2% for design purposes. Accordingly, when producing a superconducting magnet from a compound superconducting wire material, it has been extremely difficult to employ a method in which the superconducting wire material is wound into a coil while applying a tension to the wire, as in the alloy superconducting wire material.
For producing a magnet from a compound superconducting wire material, it is important to avoid the introduction of strains into the compound superconducting wire material, which may occur in association with the winding process. To this end, the production process generally includes the steps of winding an unreacted compound superconducting material into a coil, subjecting the coil to a heat treatment so as to allow niobium and tin to react with each other to form an Nb3Sn compound (wind-and-react method), and then immersing an epoxy resin into interstices between adjacent windings in vacuum thereby to prevent undesired vibrations of the wire material, or to fix the coil. Therefore, the process for producing a magnet from a compound superconducting material requires specifically designed facilities for uniformly applying heat to the coil, or immersing a resin into the wire interstices under vacuum. Particularly, in order to produce a magnet for applications which require a wide-bore coil, a large heating furnace and a large vacuum coating facility are required.
It should be noted here that there is another method which allows the production of a superconducting magnet from a compound wire material, by using, instead of a conventional Nb3Sn wire material susceptible to strains, a previously heat-treated wire material and winding it into a coil, and which is known as a double pancake method. With this method, however, the tolerable limit of strains during the production process should also be 0.2% or less, in accordance with the property characteristic of the wire material to be used, and it is thus necessary for the resulting superconducting magnet to have a wide bore. This method further suffers from problems that not only the final shape the magnet is limited, but it is also difficult, if not impossible, to apply a high tension to the wire during its winding.
As described above, a compound superconducting magnet is conventionally produced by the wind-and-react method, except for the double pancake method, and, with this method, a specifically designed facility must be introduced for the thermal treatment which is necessary for transforming the starting materials into an Nb3Sn compound and winding the compound into a coil. With this method it is also necessary to carefully handle the wire after thermal treatment in order that any undue strains and stresses may not be added into the wire. In addition, this method also requires a vacuum coating device for applying an epoxy resin and fixing the wire material.
For production of a wide-bore magnet generating a strong magnetic field, there is an actual demand for the introduction of a compound superconducting wire material which is capable of generating a strong magnetic field. This applies, for example, to a wide-bore and strong field magnet for the measurement of physical properties of an object, or a superconducting magnet for particle accelerators, for linear motor cars, for electric generators, or for elementary particle detectors. However, a compound superconducting magnet prepared by the conventional method poses a number of problems: a compound superconducting magnet produced by the conventional method can not help being large; the magnet must incorporate a large-caliber superconducting wire material to withstand the magnetic force it produces, and the large-caliber wire inevitably leads to a further enlarged coil diameter; and the production of the magnet itself is difficult. Thus, use of such a compound superconducting wire material is practically impossible at present.
When a wide-bore superconducting magnet is introduced, for example, in a particle accelerator, the overall facility becomes large in scale. Therefore, it is desirable to realize a magnet which is made as compact as possible. For production of a cryocooled superconducting magnet which does not require liquid helium, it is essential for the magnet to be small in size and light in weight besides that the superconducting magnet must be easy for production. Furthermore, it would be highly desirable that the magnet can be shaped not only as a pancake but also into a complicated form appropriate for a given purpose. With regard to a wire material to be used for the magnet, it should exhibit a sufficiently high strength capable of withstanding the electromagnetic force produced by the magnet.
In view of above, it is a general object of the present invention to provide a superconducting magnet having excellent characteristics for advantageously overcoming the above-mentioned problems inherent to the conventional superconducting magnet which uses a compound superconducting wire material.
It is a more specific object of the present invention to provide a superconducting magnet which can be produced by a simple method which makes it possible to use a previously heat-treated wire material by introducing a reinforced and stabilized Nb3Sn wire material.
It is another object of the present invention to propose a compound superconducting magnet which can be made small and light, which can be formed into a relatively complicated shape, and which is capable of generating a sufficiently strong magnetic field suitable even for applications where alloy superconducting magnets have been exclusively used in the past.
It is a further object of the present invention to provide a method for producing such a superconducting magnet simply and at a low cost, from a compound superconducting wire material.
It is a still further object of the present invention to provide a method for producing a superconducting magnet simply and with easy handling, in which the conventional magnet production method applicable to alloy superconducting materials is modified in such a way as to be also applicable to compound superconducting wire materials.
The present inventors conceived for the first time that a high-strength reinforced and stabilized Nb3Sn wire material can be advantageously obtained by substituting a reinforcing, stabilizing agent such as a copper niobium or an alumina-dispersed copper for a high purity copper contained, as a stabilizing agent, in a conventional ultra-thin, multi-filamentary Nb3Sn wire material. When such a wire material is used as a material for producing a superconducting magnet, the wire can be wound into a coil in essentially the same manner as in the production of an alloy superconducting magnet, because the wire material has a strength which is sufficiently high to withstand the bending forces accompanied with the winding process.
The present invention has been accomplished based on the above-mentioned novel recognition.
According to one aspect of the present invention, there is provided a superconducting magnet comprising: a reinforced and stabilized superconducting wire material which is wound into an electromagnetic coil; said superconducting wire material comprising a compound superconducting substance, and a reinforcing, stabilizing agent which covers said compound superconducting substance.
The superconducting magnet according to the invention may include as a compound superconducting substance, for instance, Nb3Sn, or Nb3Sn supplemented with titanium or tantalum. Furthermore, the reinforcing and stabilizing agent may comprise copper niobium or alumina-dispersed copper.
Advantageously, the reinforced and stabilized superconducting wire material takes a round or rectangular cross-section having an external dimension of 0.3 to 2 mm.
According to another aspect of the present invention, there is provided a method for producing a superconducting magnet, which comprises the steps of: preparing a reinforced and stabilized wire material comprising a raw material for a compound superconducting substance, and a reinforcing, stabilizing agent which covers said raw material; subjecting said reinforced and stabilized wire material to a preliminary heat treatment so that the wire material is transformed into a superconducting wire material; and winding said superconducting wire material into a coil.
With the production method according to the present invention, it is preferable for the reinforced and stabilized superconducting wire material to be a round or rectangular wire with an external dimension ranging from 0.3 to 2 mm, for the individual windings of the wire material to be bound together through a coat of an epoxy resin during the winding process, and for the wire material to be applied with a tension which is controlled properly during the winding process.